1. Field of the Invention
The present invention relates to apparatus and methods for providing image stabilization in high-performance optical systems.
2. Description of the Prior Art
Many optical systems used in manufacturing today are high-performance systems in which high-resolution images are formed. The images usually need to be imaged onto a light-sensitive medium, such as a photosensitive workpiece or a detector, with great precision.
An example of a high-performance optical system is a microlithographic projection lens. Such lenses are used in lithography systems, which are used to fabricate semiconductor devices such as microcircuits. Microlithographic projection lenses typically are required to resolve resolution-limit xe2x80x9ccritical dimensionxe2x80x9d features on a mask. Further, the images need to be accurately located on the wafer, i.e., to within nanometers of existing features.
In a lithography system, the position where a mask image (referred to as the xe2x80x9caerial imagexe2x80x9d) is formed in the image plane can vary due to refractive index variations of the gas (e.g., air) within the spaces between the lens elements for certain types of microlithographic lenses. The index variations can be caused by lens heating, which can arise from a number of sources such as electrical and mechanical elements within the lithography system (e.g., the wafer stage linear motors). If not properly dissipated, such heat can cause a negative thermal gradient across the housing of the lens, wherein the top of the lens is cooler than the bottom of the lens. This, in turn, can cause convective heat transfer within the lens, resulting in unstable air motion within the lens and thus variations in refractive index along the optical path. If the space between lens elements is relatively large (which is the case for many catoptric and catadioptric microlithographic lens designs), such refractive index variations can significantly alter the optical path of the light rays. This, in turn, can cause slight displacements in the image at the image plane, which can lead to alignment and/or stage positioning errors, which ultimately degrade the quality of the device being fabricated.
The present invention relates to apparatus and methods for providing image stabilization in high-performance imaging systems.
Accordingly, a first aspect of the invention is an apparatus for providing image stabilization for a high-performance optical system. The apparatus includes a heating element arranged on an upper surface of a housing of the optical system. The heating elements provide heat to one or more gas-filled spaces between lens elements of the optical system. An insulating layer is arranged over at least a portion of the housing and facilitates the heating of gas present within the one or more spaces. The heating is performed so as to create a stable thermal environment within the optical system, wherein the warmer air resides closest to the upper portion of the housing and the cooler air resides closest to the lower portion of the housing.
A second aspect of the invention is that as described immediately above and further including a gas manifold is arranged adjacent the lower surface of the housing. The gas manifold is used to flow gas around the lower surface in order to prevent heating of the lower surface. The gas manifold may also be adapted to flow gas around heat-generating elements in the lithography system to prevent heat from such elements from heating the lower portion of the housing of the optical system.
A third aspect of the invention is a method of stabilizing the imaging of a high-performance optical system having one or more gas-filled spaces that are subject to thermal instability. The method includes heating the optical system so that gas in the one or more gas-filled spaces is heated so as to maintain a temperature differential within the one or more gas-filled spaces to form a stable thermal environment within the gas-filled spaces. In an example embodiment, the heating is performed on an upper surface of the optical system to avoid convective thermal instability in the one or more gas-filled spaces.